Design of immunogens capable of inducing neutralizing antibodies (NtAb) against diverse isolates of human immunodeficiency virus (HIV) remains an unmet goal in AIDS vaccines research. Multiple factors may contribute to the difficulty in generating broadly NtAb against HIV. These include conformational masking of the conserved epitopes around the receptor and coreceptor binding sites (bs), as well as occlusion by glycan moieties (""""""""glycan shield""""""""). We recently demonstrated that removal of an N-linked glycan in the C-terminal stem of the V2 loop of HIV-1 gp120 resulted in increased neutralizing sensitivity to CD4bs antibodies and the ability of the mutant Env to mediate CD4-independent infection. Immunization with this mutant Env resulted in cross-reactive NtAb responses in macaques. Based on these findings, we hypothesize that: (1) changes resulting from specific glycan modifications lead to increased stability or accessibility of conserved epitopes in the receptor/coreceptor bs;(2) greater accessibility of these conserved sites enhances their function as targets for cross-reactive NtAb responses. In this project, we propose to test these hypotheses by examining the correlation between specific glycan modifications, receptor/coreceptor binding properties and the ability of the modified Env to induce cross-reactive NtAb and to project against SHIV challenge in macaques. Specific Aims : 1. To determine the effect of specific N-linked glycan modifications on Env antigenicity, functional integrity, receptor/coreceptor usage, and immunogenity 2. To compare the immunogenicity of native and modified Env from macrophage-tropic R5 viruses that exhibit differential affinity for CD4 and sensitivity to CD4bs antibodies (from Project 1) 3. To examine the effect of glycan modifications on a panel of Env that show enhanced ability to induce cross-reactive NtAb responses (from Project 2) 4. To examine the protective efficacy of modified Env immunogens in macaque challenge models Results from these studies are likely to provide further insight for the design of immunogens capable of eliciting NtAb against diverse isolates of HIV-1.